RFC: meson support

still WIP in a few places, but an initial draft is ready

Notes:

• might not be able to ship pre-rendered manpages with this (worth looking into further)
• :manversion: macro does nothing (in the current and proposed versions)
• I am attempting to keep everything runnable without meson, see e.g. MESON_PROCESSED macro
• running tests/coverage in meson proves a bit elusive. do we want to just keep that as a makefile thing? (same question for dist tarball generation)
• currently have parity with the arch package in installed files/dirs/perms

fixes #128
closes !386 (closed)
closes !387 (closed)